Liquid chromatography is a process by which one or more compounds from a chemical mixture may be separated and identified. In order for a chromatographic column to be able to separate biological substances effectively, it is usual to pack fine particles of separation material as tightly and as uniformly as possible in the column tube Filling of the column, or column packing as it is often referred to, is normally effected by closing one end of the column with an outlet means which includes a filter element, and pumping a liquid suspension of the particles under pressure into the other end of the column. Whereas the pumped liquid is able to pass through the filter element essentially unobstructed, the particles are retained by the filter element, so as to build up a particle bed along the length of the tube. As the column tube is filled, the particles are pressed out towards the wall of the tube and the particle bed obtains a stable compaction state with the particles well distributed, this state being maintained during the whole of the filling process.
However, when the column tube has been filled with particles and pumping of the liquid suspension is terminated to enable an inlet element to be fitted to the filling-end of the tube, the stable restraining force in the particle bed is partially lost, resulting in expansion of the particle bed. Consequently, when the column tube is once again placed under pressure, disturbing heterogeneities or irregularities are liable to occur in the particle bed, such as the formation of channels and dead volumes.
The uniformity of the packing medium within the column has a significant effect on column performance. It is desired that the particles comprising the packing medium be perfectly arranged and completely homogeneous so that the transport liquid and the sample mixture move at uniform rates through the column Areas of loose packing medium create channels causing locally increased flow rates while areas that are partially plugged due to particle aggregation create eddies that retard the flow. Such local variations in the flow rate caused by non-uniform packing medium result in transport liquid mixing that degrades the column performance resulting in broadening of the peaks and a concomitant decrease the resolving capability of the chromatography apparatus.
In order to provide versatile columns, there is provided a range of columns capable of adjustable compression of separation medium contained therein. Such columns typically comprise one or two plungers that are moveable in the column tube in order to adjust the column length. Examples of prior art columns include the XK column series from GE Healthcare, and the Kronlab ECOplus columns from YMC Europe GMBH.